Clinical Bone Marrow Transplantation is now established as a practical and useful therapy for patients with certain immunodeficiency diseases, bone marrow aplasia and leukemia. The importance of donor selection by matching the donor and recipient with histocompatibility testing is clearly recognized. However, even HL-A and MLC identical donor-recipient marrow transplants frequently lead to fatal outcome of the transplantation. This symposium will focus on a number of genetic, immunobiological, microbiological and pathological aspects of bone marrow transplantation in experimental animals and in man. It will provide an extensive educational review on both clinical and basic scientific questions which must be further analysed to help develop clinical and experimental research.